Incremental printers may produce many different kinds of undesired artifacts in printed images. These mainly include:
repeating two-dimensional patterns due to dither-mask periodicity, or periodic relationships between dither and print masks;
progressively expanding or unfolding shapes arising in error diffusion; and
simple banding due to imperfectly abutted swaths, or to printing elements (nozzles, in inkjet printing) that are defective, or progressively inoperative (e.g. clogged, in inkjet printing), or incorrectly aimed.
The present invention addresses the third category, but is not primarily directed to swath abutment as such. Thus a principal target of the invention is malfunction of printing elements, although in some cases this in turn can produce a particular form of swath-abutment failurexe2x80x94and when it does, the present invention can be effective.
(a) Sources of bandingxe2x80x94In scanning incremental printers it is well known that striations along the scan axis are a pervasive problem. Early innovations attacked the production of white or light lines due to inadequately precise printing-medium-advance mechanisms, and anomalously colored lines when subtractive primary colorants were superposed in inconsistent sequence.
More recently, the development of very inexpensive techniques for fabrication of inkjet nozzle arrays exceeding two and three centimeters in length has also introduced difficulties in control of aiming at the ends of the arrays. While that particular kind of printing-element malfunction has now been considerably mitigated, it still causes small but stubborn departures of printed swath height from printing-element-array heightxe2x80x94and consequent banding.
With the improved control of end-element aiming, focus now shifts to malfunction of elements along the entire length of the array (though this may include the end elements). Artifacts due to these elements, as compared with those addressed earlier at the array ends, are quantitatively much finerxe2x80x94but so are the demands of the marketplace.
The consumer calls for progressively finer image quality, coupled with economy. Consequently a very significant problem remains in relatively subtle banding due to intermediate printing elements that are clogged, weak (e.g. due to firing-component tolerances or fatigue) or, again, mispointed.
(b) Identification of weak printing elementsxe2x80x94Some workers in this field (see e.g. the Murcia document and also U.S. Pat. No. 6,010,205 mentioned above) have concentrated upon tactics for correcting known bad nozzles, simply leaving identification of those elements to other artisans. Some workers have proposed to monitor the dot-generating mechanism to predict failurexe2x80x94as for instance in measurement of inkjet nozzle temperature (as in the Allen document) to anticipate malfunction, or in sensing inkdrops in flight (as represented by the patent of Dr. Ix).
At least one earlier effort, represented by the Borrell document mentioned above, treats printing-element failure as a systematic result of environmental factors. Borrell measures parameters of the printer environment with an eye to entirely minimizing the occurrence of element failure.
The most direct approach, however, tries to isolate and quantitatively measure the failure itselfxe2x80x94and to do so for each printing element individually. An ideal example of that approach for the inkjet environment appears in the previously mentioned Armijo document.
Armijo forms a test pattern with inkdrops from each nozzle (if functional) arrayed in a respective test group. He can then scan a sensor across each test group to detect functionality of each nozzle alone.
In his test pattern, a failed nozzle appears conspicuously as a missing dot in the overall test pattern. A weak nozzle appears as a dot of less than full, nominal saturation. A slightly misdirected nozzle, however, may be very difficult to detect from his test pattern.
Armijo""s technique can be implemented with the naked eye, but is far more powerful when performed automatically and the results applied to initiating corrective action. His strategy provides excellent detailed information about every nozzlexe2x80x94except for incorrect aiming, as noted just abovexe2x80x94but is time consuming.
(c) Related uses of sensorsxe2x80x94Many different kinds of sensor measurements are made in laboratory and bench tests, or in preparation of color-rendering systems as in the Bockman and Guo documentsxe2x80x94as distinguished from automatic measurements made in the field by operational printers. Some such lab measurements may quantify image quality.
The Guo document uses a carriage-mounted sensor to measure color averaged over an area, in color tiles, and applies spectral modeling to determine how to refine halftoning. The Bockman document is likewise addressed to preparing a product line as such, rather than to automatic operational field calibration of finished individual printers.
All such bench and lab uses of sensors are regarded as intrinsically different from routine operational calibration use of sensors in end-user facilities by a finished printer product. The field of the present invention is limited to such latter operational uses.
In general, as mentioned above, earlier approaches to operational determination of printing-element failure have set out to isolate and measure causes as such. Thus for instance the Doval document and the Subirada document both operate sensors over printed patterns to measure swath height and spacing, and then determine how to accommodate any error found.
Subirada in particular uses a bar-type pattern, and such patterns are also known (as in the Sievert and Nelson documents) for determining interpen alignments as well as imperfectionsxe2x80x94or some adverse results of broad tolerancesxe2x80x94within individual printheads. Subirada""s invention relates to banding reduction through adjustment of printing-medium advance.
To accomplish this, he performs a fixed matching of the print-medium advance to his measured swath height, or to a fraction of it. The Baker document teaches measurement of color balance with a sensor mounted on an auxiliary sensor carriage.
These earlier sensors ride on carriages which operate in the scan direction. Some of them, however, may be activated for measurements while the print-medium advance mechanism is operating.
(d) Printmode techniquesxe2x80x94It is now very well known that image quality can be improved in many ways through scanning multielement printing arrays plural times (rather than only once) over each portion of a printing medium. Although such operation sacrifices throughput, it remains appealing where highest quality is an objective.
Such plural- or multipass printmodes entail laying down in each pass of the printing array (e.g. inkjet pen) only a fraction of the total ink required in each section of the image. Any areas left unaddressed after each pass are completed by one or more later passes.
An intrinsic benefit of this type of printing is a tendency to conceal the edges of each printed swath, and also to hide light lines formed where individual printing elements or groups of elements are not marking fully. Such a tendency is inherent simply because a missing pixel row is somewhat less conspicuous when superimposed on a printed (or partially printed) row of another pass, than when seen against an unprinted (usually white) background of a printing medium.
In liquid-colorant printing systems, plural-pass operation has additional benefits. It tends to control bleed, blocking and cockle by reducing the amount of liquid that is all on the page at any given time, and in addition may facilitate shortening of drying timexe2x80x94thus at least partially offsetting the poorer throughput during the printing process itself.
The specific partial-inking pattern employed in each pass is called a xe2x80x9cprintmaskxe2x80x9d, and the way in which these different patterns add up to a single fully inked image is known as a xe2x80x9cprintmodexe2x80x9d. Heretofore, however, it has been recognized that printmodes and printmasks can themselves introduce undesired and conspicuous artifacts.
For example some printmodes use square or rectangular checkerboard-like patterns, which tend to create objectionable moirxc3xa9 effects when the patternsxe2x80x94or frequencies or harmonics generated within these patternsxe2x80x94are close to the patterns, frequencies or harmonics of interacting subsystems. As an example, such interferences may arise from dithering systems sometimes used to render an image.
In recent years major efforts have been made to mitigate problems of patterning, through introduction of randomization in the formation or selection of printmasks and dithering masks. These efforts have led in turn to realization, on the one hand, that randomized masks if stepped or xe2x80x9ctiledxe2x80x9d through an image can themselves create undesirable strange and even bizarre patterns; and on the other hand that randomness can itself be excessive, leading to conspicuous granularity in printed images.
Also it has become more clear that there are different kinds of randomnessxe2x80x94whose uncontrolled mixture can produce noticeable and undesirable inconsistencies in spatial-frequency content. Very recent innovations (particularly the several patent documents of Garcia-Reyero et al.) therefore undertake to control the degree and character of randomization employed, as well as the size of unit patterns to be repetitively tiled in an image.
Yet another difficulty appears with increasing image-quality demands: although somewhat less conspicuous in plural-pass printing, a missing row or group of rows yet can remain noticeable. This too is disadvantageous and has been addressed by Garcia-Reyero et al. through automatic reduction in usage, so-called xe2x80x9cdownweightingxe2x80x9d, of some known-weak or known-mispointed printing elements.
In dealing with thus-downweighted printing elements, recently introduced preferred technique includes automatic substitution of other elements. It remains to be seen, however, how best to identify printing elements that are weak, clogged, or incorrectly aimed.
If such elements can be found only through methods that consume undesirably large amounts of time, or colorant, or printing media, then the overall solution may yet be unacceptable. Thusxe2x80x94as to imperfectly functioning printing elements in generalxe2x80x94even given extremely sophisticated corrective techniques, a limiting factor may yet be the identifying methods.
(e) Control of advance: nominal matching to pen heightxe2x80x94Most recently attention has once again returned to the parameter that was the earliest identified cause of banding, namely the stroke of the printing-medium-advance system (and this is the focus of the present invention as well). The recent interest in control of the stroke, however, has been much more sophisticated than before.
Original solutions to primitive banding problems were simply to establish a print-medium advance that matched the nominal pen height, or to apply the colorant in plural passes for each region of an image. In the latter case, the print-medium advance would match some fraction of the nominal pen height.
This document will refer to these original solutions as providing a nominal printing-medium advance. The nominal advance is not only fixed but also lacks any basis in detection of printed characteristics, whether swath height or otherwise.
A difficulty with such settings is that in general the actual swath height departs from the nominal pen height. Therefore in general light-line or dark-line banding must necessarily result.
(f) Control of advance: fixed matching to swath heightxe2x80x94As mentioned above, the cross-referenced Subirada document instead teaches reduction of banding through matching of the printing-medium advance to measured properties of some printed specimen. The control prescribed in that document, while of great value in the art, is relatively basic: a fixed, static matching of the advance stroke to the measured swath height, or to some fraction of that height.
A philosophy implicit in that innovation was that such matching would produce an advance value which was not merely nominal but instead ideal, though fixed. One limitation of this philosophy is that each pen (or other type of marking element) has its own respective ideal advance value, and these values for all the pens in a given printer seldom match.
(g) Control of advance: fixed matching to mean of ideal valuesxe2x80x94Yet only one value can be used by the printer at any given stroke; this leaves at least some of the pens using nonideal advance values. Accordingly some products have been configuredxe2x80x94in the interest of minimizing adverse effects on image qualityxe2x80x94to select a mean of the ideal advance values for the several pens in use.
Unfortunately this approach had at least two drawbacks. First, when one of the pens was statistically an outlierxe2x80x94that is to say, one whose ideal advance value was more than two or three standard deviations from the meanxe2x80x94inclusion of that outlier advance value in the mean would shift the mean very significantly toward the outlier value.
This shifting would minimize the conspicuousness of printing with the corresponding outlier color, so that a user would find it difficult to identify the outlier pen. Accordingly even though just one pen was causing most of the problem, and even if the user wished to optimize image quality by replacing a pen with a better one, the user would not be readily able to improve image quality by such a replacement.
On the other hand, exclusion of the outlier value from the mean would strongly exacerbate the image degradation due to the corresponding pen. In this case a user who wished to accept the adverse effects of a single mismatched pen (and thereby avoid cost and inconvenience of replacement) would be needlessly disappointed by the resulting image quality.
A second drawback of using the mean value was that some pens would be counted equally in calculating the mean even though those pens might be printing in a color (yellow, for instance, or light cyan) for which banding is relatively inconspicuous in comparison with other colors; and also even though those pens might not be printing at all in the imagexe2x80x94or might not be printing much. This would be true as well even if they were not printing at all (or much) within a particular region of the image.
(h) Control of advance: fixed matching to weighted meanxe2x80x94Due to this second drawback, some products have been configured to select a weighted mean. Two types of such weighting are known: one involves weights designed into the system for the product line, based on the relative conspicuousness of printing in different colors or dilutions, or both; and the other also includes weights based on the number of inkdrops of each color/dilution fired in the immediately-last-printed segment of the image.
Even these weightings introduced adverse effects, from the equal significance attached to printing by pens being used to print dark shadowlike regionsxe2x80x94or highlight regionsxe2x80x94of an image. Banding is generally very inconspicuous in those tonal ranges, but the overall advance value was being shifted willy-nilly anyway in response to the ideal advance value of those pens.
Such a shift was not merely a harmless waste of attention, but rather in general degraded printing by pens being actually used to print midtonesxe2x80x94for which banding is most conspicuous. Since banding in the highlight and shadow colors was not perceptibly improved, image quality in general suffered a net degradation.
A further difficulty with the inkdrop-weighted means is that they respond to patterns of ink usage in a segment already printed, and use the information to modify a different segment that is to be printed in the future. In general, usage in the two segments may be totally different; hence the system grinds through many calculations only to produce a shift in advance that may be wholly counterproductive.
(i) Control of advance: fixed matching to a direct measure of ideal valuexe2x80x94It has since been suggested, as for instance by the Cluet document, that an ideal value might instead be found by measuring nonuniformity in nominally uniform-density printed tone patches or tiles. This was to be done without attempting to measure swath height as such.
Yet still it seemed to be contemplated that such an ideal advance would be a fixed value. In other words, while Cluet provides a very insightful strategy for ideal-value identification, he falls back to the slightly earlier concept of a single, fixed value when he reaches the next step of actually using that information.
(j) Control of advance: variable valuexe2x80x94The Zapata and Askeland documents depart from the general philosophy of an advance that is fixed and ideal, and instead introduce benefits of an advance that varies, and preferably varies randomly. These inventions seek to solve banding problems through variation and randomness of advance, thereby circumventing the notable difficulties of introducing variation and randomness in printmasking.
Accordingly these innovations are extremely valuable in the art. Nevertheless they lack any suggestion of obtaining the earlier-disclosed benefits of a fixed ideal advance value tooxe2x80x94that is to say, at the same time. Some specific advance distances, in point of fact, are better than others.
(k) Conclusionxe2x80x94As this discussion shows, limitations in the accuracy, speed and economy of methods for identifying failed or failing printing elements (and for correcting, accommodating or compensating for those identified elements) continue to impede achievement of uniformly excellent inkjet printing. In particular, these limitations include some problems that have been known in the field for some while but are amplified by the advent of inkjet pens two and a half centimeters (one inch) long. Thus important aspects of the technology used in the field of the invention are amenable to useful refinement.
The present invention introduces such refinement. In its preferred embodiments, the present invention has several aspects or facets that can be used independently, although they are preferably employed together to optimize their benefits.
In preferred embodiments of a first of its facets or aspects, the invention is a test pattern for determining optimum printing-medium advance in an incremental printer that uses an image-marking device which prints in a particular colorant. The pattern includes a printing medium; and, marked on the printing medium, plural representative image patches for the particular colorant.
Each of the representative image patches includes plural overlapping swaths of the colorant. Corresponding features of the overlapping swaths are spaced by a certain distance selected for each of the patches respectively. The certain distances are different for the plural patches, respectively.
The foregoing may represent a description or definition of the first aspect or facet of the invention in its broadest or most general form. Even as couched in these broad terms, however, it can be seen that this facet of the invention importantly advances the art.
In particular, the novel test pattern enables objective evaluation of image quality in its dependence upon the spacing of overlapping swathsxe2x80x94i.e., upon the advance stroke. In this way the novel test pattern facilitates determination of optimum advance distance, without any need or effort to measure printing-element array height.
Although the first major aspect of the invention thus significantly advances the art, nevertheless to optimize enjoyment of its benefits preferably the invention is practiced in conjunction with certain additional features or characteristics. In particular, if the test pattern is to be used for determining optimum advance in the printer, and the image-marking device includes plural marking units for marking in plural different particular colorants respectively, then preferably the patches include for each certain distance a set of plural patches; and each set includes at least one patch for each of the colorants.
In case the patches are in fact plural, one for each colorant as just described, then progressively nested subpreferences include the conditions that:
in each set, all the patches be adjacent to one another along a scanning direction;
plural alignment reference lines be printed in association with each set;
the alignment reference lines be above each set; and
the alignment lines extend across substantially the entire pattern.
Additional subsidiary preferences are that at least one nozzle-conditioning patch be associated with each of the representative image patches, and that nozzle-conditioning patches be adjacent to their associated representative image patches, along the scanning direction, and also that the representative image patches include area fills.
Such area fills, when used, are at different tonal levels for at least some of the different colorants, respectively. The tonal levels preferably are between twenty-five and fifty-five percent for yellow colorant, and between forty-five and seventy-five percent for at least one other full-strength colorant; and it is more strongly preferable that they be roughly forty percent for yellow colorant, and roughly sixty percent for the at least one other undiluted colorant.
Also preferably the tonal levels are between seventy-five and one hundred percent for at least some dilute colorants; and, still more particularly, roughly ninety percent for at least some dilute colorants. Another preference is that the xe2x80x9ccertain distancesxe2x80x9dxe2x80x94the distances by which the swaths are spaced apartxe2x80x94be distributed about a nominal value for the advance distance.
In preferred embodiments of its second major independent facet or aspect, the invention is a method of determining optimum printing-medium advance in an incremental printer that uses image-marking devices which print in respective different colors or color dilutions. The method includes the step of printing a test pattern that includes a set of representative image patches at each of plural printing-medium advance settings in turn.
Each set includes at least one representative image patch for each of the different colors or color dilutions. The method also includes the step of performing optical measurements of the test pattern to ascertain a relationship between the printing-medium advance settings and resulting image quality of the patches.
The foregoing may represent a description or definition of the second aspect or facet of the invention in its broadest or most general form. Even as couched in these broad terms, however, it can be seen that this facet of the invention importantly advances the art.
In particular, this aspect of the invention makes it possible to evaluate the advance-stroke setting for several pens or other marking devices all in a single procedure. This is particularly noteworthy because this method corresponds to actual printing of color imagesxe2x80x94i.e., printing with a number of marking devices all at once.
Although the second major aspect of the invention thus significantly advances the art, nevertheless to optimize enjoyment of its benefits preferably the invention is practiced in conjunction with certain additional features or characteristics. For example, preferably the method also includes the step of, in association with each representative image patch or set, printing either a nozzle-conditioning patch or an alignment reference line.
In preferred embodiments of its third major independent facet or aspect, the invention is an incremental printer for using image-marking devices to form images on a printing medium. The printer includes a support for the printing medium, and also a carriage for holding the marking devices and scanning the marking devices relative to the medium, to form images on the medium.
Further included is a printing-medium advance mechanism for progressively moving the medium relative to the carriage at right angles to the scanning. The printer also includes a sensor for measuring test-pattern image quality.
In addition the printer includes some means for performing certain control and operational functions. These means involve a programmed processor. It will be understood, however, that the program may be incorporated into such a processor in the course of manufacturexe2x80x94as is familiar for devices of the type known as xe2x80x9capplication-specific integrated circuitsxe2x80x9dxe2x80x94rather than being literally programmed into the processor later.
For breadth and generality in discussing the invention, these means will be called the xe2x80x9cprogrammer processor meansxe2x80x9d. One function of these means is controlling the carriage, the advance mechanism and the marking devices to print a test pattern.
The test pattern includes a set of representative image patches at each of plural printing-medium advance settings in turn. Each set includes at least one representative image patch for each of plural different colors. Another function of the controlling and operating means is operating the sensor and interpreting resulting signals from the sensor to determine optimum printing-medium advance.
The foregoing may represent a description or definition of the third aspect or facet of the invention in its broadest or most general form. Even as couched in these broad terms, however, it can be seen that this facet of the invention importantly advances the art.
In particular, this aspect of the invention is a piece of apparatus that can automatically obtain the benefits of the first two invention facets, discussed above. That is, this third aspect of the invention can automatically determine the best advance stroke that it can use with the particular set of marking devices that is actually in place.
Although the third major aspect of the invention thus significantly advances the art, nevertheless to optimize enjoyment of its benefits preferably the invention is practiced in conjunction with certain additional features or characteristics. In particular, preferably the invention also includes the image-marking devices; and preferably these include inkjet pens.
In addition the programmed processor means preferably further include some means for determining which particular marking device is most active in a particular swath of a desired image; and also determining an optimum medium advance for at least the particular marking device. The same means are also used for employing the optimum advance for that device in printing the particular swath.
An alternative preference is that the programmed processor means include means for determining the relative degree of activity of each marking device, respectively, in a particular swath of a desired image; and taking that relative degrees of activity into account in determining an optimum medium advance for all the marking devices considered in the aggregate, at the particular swath. The same means are also used for employing the optimum advance in printing the particular swath.
Certain contextual features, and certain elements of the inventive combinations themselves, are common to preferred embodiments of the fourth, fifth, sixth and seventh major independent facets or aspects of the invention. As to the common environmental or contextual features, in preferred embodiments of these four facets the invention is an incremental printer for using image-marking devices to form an image on a printing medium.
As to the elements of the inventive combinations themselves, the common elements of the printer include a support for the printing medium.
They also include a carriage for holding the marking devices and scanning the marking devices relative to the medium, to form the image on the medium. The common elements of the printer also include a printing-medium advance mechanism for progressively moving the medium relative to the carriage at right angles to the scanning.
Also included is a sensor for measuring test-pattern image quality; and means for performing certain operating and control functions. Although the specific functions are different from those mentioned earlier for the third facet of the invention, these means too will be called simply xe2x80x9cprogrammed processor meansxe2x80x9d.
The functions of the programmed processor means differ as among these four aspects of the invention. The different functions are specified below.
In preferred embodiments of the fourth independent facet of the invention, the functions of the programmed processor means comprise these three:
operating the sensor and interpreting resulting signals from the sensor to determine optimum printing-medium advance;
thereafter controlling the carriage, the advance mechanism, and the marking devices to employ particular printing-medium advance values while printing the image; and
selecting the particular advance values to provide a sequence of values that varies about the determined optimum advance.
The foregoing may represent a description or definition of the fourth aspect or facet of the invention in its broadest or most general form. Even as couched in these broad terms, however, it can be seen that this facet of the invention importantly advances the art.
In particular, this form of printing-medium advance control is the first to achieve in combination the banding-reduction benefits of optimization with those of variation. More specifically, the optimization benefits may predominate for those pens whose ideal advance is especially close to the overall optimum stroke, while the variation benefits may predominate for those pens whose ideal advance is more remote.
Although the fourth major aspect of the invention thus significantly advances the art, nevertheless to optimize enjoyment of its benefits preferably the invention is practiced in conjunction with certain additional features or characteristics. In particular, preferably the sequence of values is a pseudorandom sequence perturbed to preferentially include values relatively nearer to the determined optimum advance.
Thereby the banding-reduction benefits of randomization in printing-medium advance are added to those of variation per se and those of optimization, discussed above. For this case, in one particularly preferable implementation the sequence of values is obtained by combination of a normal distribution with a pseudorandom number generator, substantially according to the function AP exhibited belowxe2x80x94AP being the printing-medium advance and AO a nominal advance value.
AP(f1|all 1)xe2x89xa1
AOxc2x7NORMAL[fM(rand), 
"sgr"f]
with the distribution truncated to the lower and upper PBF values, where   NORMAL  ≡      μ    +                  σ        2            ⁢                                                  -                              σ                2                                      ·                          ln              ⁡                              [                                  rand                  ⁡                                      (                    1                    )                                                  ]                                                    ·                  cos          ⁡                      [                          2              ⁢              π              ⁢                              xe2x80x83                            ⁢                              rand                ⁡                                  (                  1                  )                                                      ]                              
"sgr"xe2x89xa1"sgr"f, standard deviation of all f1       μ    ≡          f      M        ≡                            ∑                      i            =            1                    N                ⁢                  xe2x80x83                ⁢                  f          i                    N        ,
Nxe2x89xa1number of pens, maximum value of i
rand(x)xe2x89xa1a function that generates uniformly distributed random numbers from 0 through x   NORMAL  ≡            f      M        +                            σ          f                2            ⁢                                                  -                              σ                f                2                                      ·                          ln              ⁡                              [                                  rand                  ⁡                                      (                    1                    )                                                  ]                                                    ·                  cos          ⁡                      [                          2              ⁢              π              ⁢                              xe2x80x83                            ⁢                              rand                ⁡                                  (                  1                  )                                                      ]                              
so       A    p    ≡            A      O        ⁡          (                        f          M                +                                            σ              f                        2                    ⁢                                                                      -                                      σ                    f                    2                                                  ·                                  ln                  ⁡                                      [                                          rand                      ⁡                                              (                        1                        )                                                              ]                                                                        ·                          cos              ⁡                              [                                  2                  ⁢                  π                  ⁢                                      xe2x80x83                                    ⁢                                      rand                    ⁡                                          (                      1                      )                                                                      ]                                                        )      
(truncated as mentioned above).
In preferred embodiments of its fifth major independent facet or aspect, the programmed processor means are for performing these functions, rather than those listed above for earlier-discussed facets of the invention:
operating the sensor and interpreting resulting signals from the sensor to determine an optimum printing-medium advance for each marking device respectively; and
thereafter controlling the carriage, the advance mechanism, and the marking devices to employ a particular printing-medium advance value that is substantially the median of the optimum advances for the image-marking devices respectively.
The foregoing may represent a description or definition of the fifth aspect or facet of the invention in its broadest or most general form. Even as couched in these broad terms, however, it can be seen that this facet of the invention importantly advances the art.
In particular, in comparison with using a mean of the optimum advances, this aspect of the invention helps to identify a single pen or other image-marking device that is least compatible with the others. This capability is valuable because it enables a user to obtain a significant image-quality improvement simply by replacing that one device.
Although the fifth major aspect of the invention thus significantly advances the art, nevertheless to optimize enjoyment of its benefits preferably the invention is practiced in conjunction with certain additional features or characteristics. In particular, preferably this fifth facet of the invention can be practiced in conjunction with other aspectsxe2x80x94for example the above-mentioned variation of advance, in this case a variation about the median.
In preferred embodiments of its sixth major independent facet or aspect, the programmed processor means are for performing these functions:
operating the sensor and interpreting resulting signals from the sensor to determine an optimum print-medium advance for each marking device respectively;
thereafter determining, for a specific image swath, the image density contributed by each marking device respectively; and
thereafter controlling the carriage, the advance mechanism, and the marking devices to employ a particular printing-medium advance value that is substantially a sensitivity-weighted mean of the optimum advances for the image-marking devices respectively.
The sensitivity-weighted mean is calculated substantially by weighting an optimum advance value for each marking device by the sensitivity of banding to printing density for a color in that image-marking device respectively.
The foregoing may represent a description or definition of the sixth aspect or facet of the invention in its broadest or most general form. Even as couched in these broad terms, however, it can be seen that this facet of the invention importantly advances the art.
In particular, use of sensitivity weighting introduces a far more sophisticated and effective advance-selection criterion than known heretofore. For instance, earlier weighting schemes revolved about merely a numerical counting of colorant quantities dispensed from each pen respectively.
Such a sheerly numerical counting is vulnerable to excessive emphasis on the effects of pens that are not printing color tones which are susceptible to serious banding problems. The present invention emphasizes exactly those tones, for reference in determining ideal advance stroke, and thereby makes improvements that are readily perceivable.
Although the sixth major aspect of the invention thus significantly advances the art, nevertheless to optimize enjoyment of its benefits preferably the invention is practiced in conjunction with certain additional features or characteristics. In particular, preferably the specific image swath is a swath that is prospectively to be printed; and the particular printing-medium advance value is employed for the specific image swath prospectively to be printed. Further the sensitivity-weighted mean is preferably found substantially according to the expression exhibited below.
AP(fi|all i)xe2x89xa1
AO fMWS 
where       f    MWS    ≡            ∑                                    S            i                    ⁢                      (                          ρ              i                        )                          ⁢                  W          i                ⁢                  f          i                            ∑                                    S            i                    ⁢                      (                          ρ              i                        )                          ⁢                  W          i                          A    p    ≡            A      O        ⁢                  ∑                              S            i                    ⁢                      W            i                    ⁢                      f            i                                      ∑                              S            i                    ⁢                      W            i                              
In preferred embodiments of its seventh major independent facet or aspect, the programmed processor means are for performing these functions:
operating the sensor and interpreting resulting signals from the sensor to determine an optimum printing-medium advance for each marking device respectively;
thereafter determining, for a specific image swath that is prospectively to be printed, a characteristic of the image components to be contributed by each marking device respectively; and
thereafter, for the specific image swath that is prospectively to be printed, controlling the carriage, the advance mechanism, and the marking devices to employ a particular printing-medium advance value that is a function of the determined characteristic of image components to be contributed by each device respectively.
The foregoing may represent a description or definition of the seventh aspect or facet of the invention in its broadest or most general form. Even as couched in these broad terms, however, it can be seen that this facet of the invention importantly advances the art.
In particular, here the invention selects the advance stroke based on portions of the image that are to be printed, in the future, with that selected strokexe2x80x94rather than based upon portions that have already been printed, and for which it is too late to choose a suitable stroke.
Although the seventh major aspect of the invention thus significantly advances the art, nevertheless to optimize enjoyment of its benefits preferably the invention is practiced in conjunction with certain additional features or characteristics. In particular, preferably the specific image swath is to be printed substantially immediately.
All of the foregoing operational principles and advantages of the present invention will be more fully appreciated upon consideration of the following detailed description, with reference to the appended drawings, of which: